Wide Band Gap Semiconductors
Boron nitride is naturally p-type an alternate for realizing hole injection. Realizing optoelectronic devices that combine wurtzitic nitrides with a hexagonal crystal is however a multiple hurdle race. The recent growth of h-BN films by metal-organic vapour epitaxy without PL at 5.5 eV is an indicator of a really very good quality. This indicates that the quality of h-BN epilayers is now mastered and that this material is in its way for application in the UV.
While discovering in 1986 that the growth of a low temperature buffer layer at the interface with the sapphire substrate was improving the performances of GaN, Nobel Prizes Isamu Akasaki and Hiroshi Amano, as well as their collaborators of Nagoya probably did not realize how impacting would be this discovery.
A couple of years later, after that they had managed to control both p-type and n-type dopings, they realized the first GaN-AlGaN light emitting diode (LED) . Hundreds of different devices derive from their pioneering works at the Meijo and Nagoya universities.
The opto-electronic market now overlaps with the huge market of solid state lighting, based on the utilization of blue light emitters coupled to yellow light emitting phosphors.
Although the yield of such LEDs is very good in the blue, the supremacy of the nitride technology is more difficult to establish at both sides, namely in the green and ultra violet (UV) wavelength regions. The reasons for these difficulties have different origins. In case of devices that would fill the green gap, it is mandatory to grow very high quality indium-rich alloys so that non radiative recombination rates are almost eradicated and no longer compete against the radiative recombination rate. Regarding the deep UV, the dominating issue is the control of the p-type doping of aluminium-rich AlGaN alloys and their valence band physics.
Boron nitride is an alternate for realizing hole injection. Realizing optoelectronic devices that combine wurtzitic nitrides with a hexagonal crystal is however a multiple hurdle race. Researchers are facing many challenges but the recent growth of h-BN films by metal-organic vapour epitaxy (MOVPE) (Appl. Phys. Lett. 108, 052106, 2016) indicates that scientists are probably on the good way.
In this communication we demonstrate the indirect nature of the band gap of h-BN using one-photon and two-photon cw photoluminescence, photoluminescence excitation spectroscopy and time-resolved photoluminescence in the 8K-300K range, using bulk h-BN crystals commercialized by hqgraphene (www.hqgraphene.com). We demonstrate the specificity of the indirect band gap (valence band maximum at K point of the first Brillouin zone, and conduction band minimum at M) and we identify different phonon-assisted transitions and their overtones. The indirect exciton binding energy is found to be about 130 meV (Nature Photonics 10.1038/nphoton.2015.277). We demonstrate that the strong, lower energy (about 5.5 eV) luminescence lines result of a cascade of TO(K) phonon-assisted intervalley scattering processes stimulated by defects of the crystal (Phys. Rev. B 93, 035207,2016).
This permits us to claim quantitatively, based on our study that the absence of such PL at 5.5 eV in the MOVPE crystals referred above is an indicator of a really very good quality. This indicates, we believe it: i) that the quality of h-BN epilayers is now mastered and ii) that it is interesting and iii) that this material is in its way for application in the UV. Time has now come for it to be more deeply investigated, in particular by device growers.